The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
A fuel control system reduces emissions of a gasoline engine. The fuel control system controls an amount of fuel delivered to the engine based on data sensed by one or more exhaust gas oxygen (EGO) sensors disposed in an exhaust system of a vehicle. The EGO sensors are of two types: universal (wide-range) EGO sensors and switching-type EGO sensors. Typically, the term EGO sensor refers to a switching-type EGO sensor. As used herein, EGO sensors include wide-range EGO sensors and switching-type EGO sensors unless specified otherwise.
The fuel control system may include an inner feedback loop and an outer feedback loop. The inner feedback loop may use data from an EGO sensor arranged before a catalytic converter (i.e., a pre-catalyst EGO sensor) to control the amount of fuel delivered to the engine. For example, when the pre-catalyst EGO sensor senses a rich air/fuel ratio in an exhaust gas (i.e., low net oxygen), the inner feedback loop may decrease a desired amount of fuel sent to the engine (i.e., decrease a fuel command). When, however, the pre-catalyst EGO sensor senses a lean air/fuel ratio in the exhaust gas (i.e., excess net oxygen), the inner feedback loop may increase the fuel command. This maintains the air/fuel ratio near true stoichiometry, thereby improving the performance of the fuel control system. Improving the performance of the fuel control system may improve fuel economy of the vehicle.
The inner feedback loop may use a proportional-integral control scheme to correct the fuel command. The fuel command may be further corrected based on a short term fuel trim or a long term fuel trim. The short term fuel trim may correct the fuel command by changing gains of the proportional-integral control scheme based on engine operating conditions. The long term fuel trim may correct the fuel command when the short term fuel trim is unable to fully correct the fuel command within a desired time period.
The outer feedback loop may use information from an EGO sensor arranged after the converter (i.e., a post-catalyst EGO sensor) to correct the EGO sensors and/or the oxygen storage state of the converter when there is an unexpected reading. For example, the outer feedback loop may use the information from the post-catalyst EGO sensor to maintain the post-catalyst EGO sensor at a required voltage level. As such, the converter maintains a desired amount of oxygen stored, thereby improving the performance of the fuel control system. The outer feedback loop may control the inner feedback loop by changing thresholds used by the inner feedback loop to determine whether the air/fuel ratio is rich or lean.
Exhaust gas composition affects the behavior of the EGO sensors, thereby affecting accuracy of the EGO sensor values. For example, an EGO sensor may indicate that an exhaust gas includes a rich air/fuel ratio when the exhaust gas actually does not include the rich air/fuel ratio. As a result, fuel control systems have been designed to operate based on values that are different than those reported. For example, fuel control systems have been designed to operate “asymmetrically,” where the threshold used to indicate the lean air/fuel ratio is different than the threshold used to indicate the rich air/fuel ratio.
Since the asymmetry is a function of the exhaust gas composition, and the exhaust gas composition is a function of the engine operating conditions, the asymmetry is typically designed as a function of the engine operating conditions. The asymmetry is achieved indirectly by adjusting the gains and the thresholds of the inner feedback loop, which requires numerous tests at each of the engine operating conditions. Moreover, this extensive calibration is required for each powertrain and vehicle class and does not easily accommodate other technologies, including, but not limited to, variable valve timing and lift.